The present invention relates to liquid crystal display devices, and, more particularly, to a liquid crystal display device having a drive means for applying a voltage to each pixel of a liquid crystal display device, and an illumination light source.
The liquid crystal display device (hereinafter referred to as an LCD) produces a highly precise display and has characteristics such as a low consumed power, reduced volume for the display device, or the like. It is expected that liquid crystal display devices will entirely replace a cathode ray tube (hereinafter referred to as a CRT) in various usages, such as a computer monitor, a television display device, or the like. However, since the LCD does not have sufficient image quality in displaying a moving picture as compared with the CRT, improvement in the quality of the moving picture is desired. In particular, it is required that the moving picture can be displayed with a high image quality on the basis of the current television signal at the time of the application of the LCD to a television display device.
It is assumed that problems in the moving picture display of the LCD lie in the following points. In the beginning, in the case where a screen is displayed which shows a white object 50 moving against a black background in a direction of an arrow as shown in FIG. 20(a), an xe2x80x9cobject blurxe2x80x9d is generated in which a contour of an object 50 can be perceived in a blurred manner by an observer as shown in FIG. 20(b). In addition, a xe2x80x9cghostxe2x80x9d is also generated in which a residual image 51 of the object 50 before the movement can be perceived as shown in FIG. 20(c).
One problem in such a moving picture display results from a long response time of the liquid crystal with respect to the signal. In the LCD of the twisted nematic type (hereinafter referred to as a TN type) and the super twisted nematic type (hereinafter referred to as a STN type) which are currently generally used, electro-optic response of the liquid crystal is relatively slow so that it takes a long time from the application of an electric field to the attainment of a desired light transmittance with the electrically changed arrangement of the liquid crystal molecule and is several times longer than 16.7 msec, which is a display cycle of one screen (hereinafter referred to as one frame) in a ordinary image signal. Consequently, as shown in FIG. 21, even when a voltage for a white display is applied to a liquid crystal which is providing a black display, a relatively long time is required until the liquid crystal attains a completely white state. Thus, an optical response of the liquid crystal at the moving potion is not completed in one frame period. A delay in the optical response of this liquid crystal is visually recognized as a motion blur and a ghost.
Furthermore, it is considered that the fact that displaying in the LCD is of a hold type, in which light emission of same amount continues until the LCD is rewritten by image signal of the next frame, results in a low display image quality with respect to the moving picture. In a thin film transistor type (hereinafter referred to as the TFT type) LCD which is mainstream among LCDs, electric charge for applying electric field to the liquid crystal can be held at a relatively high ratio until the electric field is subsequently applied. Consequently, as shown in FIG. 22(a), each of the pixels of the LCD continuously transmits light until the pixel is rewritten with the application of the electric field on the basis of the image signal of the next frame. On the other hand, in the CRT display device which provides a display by scanning a screen with an electron beam to allow fluorescent material on the screen to emit light, as shown in FIG. 22(b), light emission of each pixel is an impulse-like manner. Consequently, the LCD has a low time frequency characteristic of the image display light as compared with the CRT, so that the spatial frequency characteristic is lowered along with this to provide a blur in a visually observed image.
There is disclosed, for example, in the Japanese Unexamined Patent Publication No. 11-202285 an example in which a backlight is equipped with plurality of lamps and the lamps are sequentially driven in order to improve the image quality in the display of the moving picture of the LCD. FIG. 23 is a block diagram showing a structure of such liquid crystal display device. A backlight 54 arranged on a rear surface of the liquid crystal panel is divided into a plurality of light emission regions 54a through 54d, so that a lamp 56 in each of the light emission regions 54a through 54d is allowed to be subsequently emitted with a lighting control circuit 60 while holding a definite time delay with respect to the operation of writing an image to the liquid crystal in a corresponding region.
FIG. 24 is a timing chart showing a relation between an optical response of the liquid crystal and the backlight emission in such liquid crystal display device. In FIG. 24, a signal for each pixel, an optical response of the liquid crystal in each pixel, and turn ON/OFF timing of the lamps in the backlight are shown.
In the beginning, at the previous frame, transmittance 64 of the pixel in the n-th row is rewritten from black, i.e., lower transmittance, to white, i.e., higher transmittance, by applying a voltage corresponding to a white signal. Immediately after rewriting, the transmittance 64 of the pixel increases rapidly and then increases gradually toward a truly white display, taking the time of several frames. In the subsequent frame, transmittance 65 of a pixel in the (n+I)-th row is rewritten from black to white with the same behavior as the pixel in n-th row in a delay of one frame period (about 16 msec).
At the same time, the backlight is lit only in a predetermined period after the lapse of a definite time from the rewriting of the image signal in each frame period as shown in the lower part of FIG. 24. As a consequence, the halfway transition in the transmittance of the liquid crystal is not apparent to observers so that the image quality in displaying the moving picture is improved. Furthermore, the transmitted light of each pixel comes close to the impulse-like manner, so that the image quality in the moving picture display is improved.
However, in the conventional liquid crystal display which has been explained above, the motion blur is suppressed but the xe2x80x9cghostxe2x80x9d cannot be sufficiently erased. As shown in FIG. 20(c), the xe2x80x9cghostxe2x80x9d appears as a difference in contrast between the region 52 which is rewritten from the black image to the white image and the region 53 which is rewritten from the white image to the white image. That is, since response of the liquid crystal is relatively slow, the region 52 recently rewritten to white is darker than the region 53 anciently rewritten to white. Although illumination by the backlight is limited to the end of each frame period, transmittance 64 of the liquid crystal in the region 52 which is rewritten from black to white and transmittance 66 of the liquid crystal in the region 53 which is rewritten from white to white are different even in this illuminating period as shown in FIG. 24 because response time of the general TN-type liquid crystal is several times longer than the frame period. This luminance difference completely disappears several frames after the rewriting of image. Consequently, the xe2x80x9cghostxe2x80x9d remains even when the lighting period of the backlight is restricted to the shortest possible level.
Furthermore, as has been already explained in FIG. 21, the response of the liquid crystal is relatively slow, so that several frame periods are required until the approximate completion of the response. For all this, in the conventional liquid crystal display device, a voltage is applied to the liquid crystal which produces a desired transmittance in the state in which a sufficient time passes and the response of the liquid crystal is approximately completed. As a consequence, the transmittance of the liquid crystal does not attain a desired transmittance during the current frame, so that the display quality of the moving picture is deteriorated.
Therefore, the present invention provides a liquid crystal display device which can eliminate the xe2x80x9cghostxe2x80x9d even when using the TN-type liquid crystal having a slow response rate and which can obtain a favorable display quality of the moving picture by compensating for the slow response of the liquid crystal.
Furthermore, an object of the present invention is to provide a liquid crystal display device which has a high response rate of the liquid crystal and an excellent display performance of the moving picture without remarkably increasing the required amount of the memory and the scale of the circuit.
In order to solve the above problem, a liquid crystal display device according to one aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period and providing the corrected image signal to the horizontal driving means, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, is provided.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame, is provided,
wherein an election means, for selectively providing the corrected image signal or an erasure signal to the horizontal driving means in a manner that the corrected image signals are provided for the pixels in even number rows while the erasure signal is provided for the pixels in odd number rows at even number frames and the erasure signal is provided for the pixels in even number rows while the corrected image signals are provided for the pixels in odd number rows at odd number frames, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, is provided.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a temperature detecting means for detecting temperature of liquid crystal in the display panel is provided,
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period using said detected temperature as a parameter and providing the corrected image signal to the horizontal driving means, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, is provided.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a temperature detecting means for detecting temperature of liquid crystal in the display panel is provided,
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period using said detected temperature as a parameter, is provided,
wherein an election means, for selectively providing the corrected image signal or an erasure signal to the horizontal driving means in a manner that the corrected image signals are provided for the pixels in even number rows while the erasure signal is provided for the pixels in odd number rows at even number frames and the erasure signal is provided for the pixels in even number rows while the corrected image signals are provided for the pixels in odd number rows at odd number frames, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, is provided.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period and providing the corrected image signal to the horizontal driving means, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, and current flowing through a lamp in each light emitting region is independently controlled with each other, is provided.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period and providing the corrected image signal to the horizontal driving means, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, and turn on period of each light emitting region is independently controlled with each other, is provided.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and switching means connected to each of the pixels;
a vertical driving circuit for scanning the whole display area of the display panel over one frame period by selecting the rows of pixels alternately while turning on the switching means connected thereto; and
a horizontal driving means for applying voltage, which corresponds to an image signal, to each pixel in said selected row through the switching means being turned on;
wherein a signal correcting means, for correcting a level of an original image signal to a level with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period and providing the corrected image signal to the horizontal driving means, is provided, and
wherein an illumination device for illuminating the display panel with a plurality of light emitting regions thereof, said light emitting regions sequentially turns on and off in synchronization with the selection of rows belonging to each region while holding a definite time delay to the selection of rows, and turn on period of each light emitting region is further divided into turn on sub-periods and turn off sub-periods, is provided.
It is preferable that a ratio of the turn on sub-periods to the turn on period for each light emitting region is independently controlled with each other.
Furthermore, the above liquid crystal display device according to the present invention is characterized in that the erasure signal is either an image signal of black level or an image signal of intermediate gray level.
Furthermore, a liquid crystal display device according to another aspect of the present invention is characterized in that an image signal of current frame is externally input, a voltage with which transmittance designated by said current frame image data is attained within one frame period is applied to liquid crystal at the current frame, and said voltage applied to the liquid crystal varies in accordance with temperature of liquid crystal.
Furthermore, a liquid crystal display device according to another aspect of the present invention is characterized in that a voltage with which transmittance designated by a current frame image data is attained within one frame period is determined depending on the current frame image data and a previous frame image data and applied to liquid crystal at the current frame, and said voltage applied to the liquid crystal varies in accordance with temperature of liquid crystal.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a temperature detection circuit for detecting a temperature of a liquid crystal;
a frame memory for storing a present frame image signal for a definite time to output as a previous frame image signal;
a plurality of signal conversion tables in which output data is stored in correspondence to the each value of the previous frame image signal and each value of the current frame image signal; and
a processor for determining the output data from the current frame image signal and the previous frame image signal by using one of the signal conversion table selected on the basis of the detected temperature of the temperature detection circuit.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a temperature detection circuit for detecting a temperature of a liquid crystal;
a frame memory for storing a present frame image signal for a definite time to output as a previous frame image signal;
a plurality of signal conversion tables in which output data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal; and
a processor for determining the output data from the current frame image signal and the previous frame image signal by using one of the signal conversion table selected on the basis of the detected temperature of the temperature detection circuit.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a temperature detection circuit for detecting a temperature of a liquid crystal;
a converting means for converting a bit length of the current frame image signal;
a frame memory for storing a present frame image signal, having bit length thereof converted, for a definite time to output as a previous frame image signal;
a plurality of signal conversion tables in which output data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal; and
a processor for determining the output data from the current frame image signal and the previous frame image signal by using one of the signal conversion table selected on the basis of the detected temperature of the temperature detection circuit.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a temperature detection circuit for detecting a temperature of a liquid crystal;
a frame memory for storing a present frame image signal for a definite time to output as a previous frame image signal;
a plurality of signal conversion tables in which output data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal;
a signal conversion interpolation table in which interpolation differential data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal; and
a processor for determining the output data from the current frame image signal and the previous frame image signal by using one of the signal conversion table selected on the basis of the detected temperature of the temperature detection circuit and the signal conversion interpolation table.
Furthermore, a liquid crystal display device according to another aspect of the present invention comprises:
a temperature detection circuit for detecting a temperature of a liquid crystal;
a converting means for converting a bit length of the current frame image signal;
a frame memory for storing a present frame image signal, having bit length thereof converted, for a definite time to output as a previous frame image signal;
a plurality of signal conversion tables in which output data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal;
a signal conversion interpolation table in which interpolation differential data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal; and
a processor for determining the output data from the current frame image signal and the previous frame image signal by using one of the signal conversion table selected on the basis of the detected temperature of the temperature detection circuit and the signal conversion interpolation table.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a converting means for converting a bit length of the current frame image signal;
a frame memory for storing a present frame image signal, having bit length thereof converted, for a definite time to output as a previous frame image signal;
a signal conversion table in which output data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal;
a processor for determining the output data from the current frame image signal and the previous frame image signal by using the signal conversion table; and
an illumination device for illuminating the display area of the liquid crystal display with a plurality of horizontal stripe light emitting regions thereof separately.
Furthermore, the liquid crystal display device according to another aspect of the present invention comprises:
a temperature detection circuit for detecting a temperature of liquid crystal;
a converting means for converting a bit length of the current frame image signal;
a frame memory for storing a present frame image signal, having bit length thereof converted, for a definite time to output as a previous frame image signal;
a plurality of signal conversion tables in which output data is stored in correspondence to some of each value of the previous frame image signal and some of each value of the current frame image signal;
a processor for determining the output data from the current frame image signal and the previous frame image signal by using one of the signal conversion tables selected on the basis of the detected temperature of the temperature detection circuit; and
an illumination device for illuminating the display area of the liquid crystal display with a plurality of horizontal stripe light emitting regions thereof separately.
Furthermore, in the above liquid crystal display device according to the present invention, the number of gradations represented by the previous frame image signal having bit length thereof converted is preferably equal to the number of gradations of the previous frame image signal in the signal conversion table.
Furthermore, in the liquid crystal display device according to the present invention, the output data in the signal conversion table is previously determined preferably in a manner that a transmittance designated by the current frame image signal is attained within one frame period by applying a voltage determined by the output data.
Furthermore, another aspect of the present invention relates to a liquid crystal display device of an active matrix type for displaying an image signal of interlaced type comprising even number fields and odd number fields, wherein original image signal designating a image to be displayed is corrected so as to enlarge a level difference between the original image signal and an erasure signal, and corrected image signals are provided for the pixels in even number rows while a erasure signal is provided for the pixels in odd number rows at even number fields and the erasure signal is provided for the pixels in even number rows while corrected image signals are provided for the pixels in odd number rows at odd number fields.
Since the image displayed in the previous field is erased by writing the erasure signal before writing the image signal, the allowed time for optical response of each pixel can be uniformed irrespective of the display image of the previous frame. For example, in the case where the pixel for providing the black display and the pixel for providing the white display in the previous frame are rewritten in a new gradation level in the same frame, any of the pixels is uniformed in the same erasure signal in the even number field and the odd number field, followed by being rewritten in the gradation signal in the next field. Consequently, a luminance difference between pixels resulting from the difference in the response of the liquid crystal can be virtually eliminated. Consequently, the xe2x80x9cghostxe2x80x9d can be erased.
In order to conduct the above operation, the liquid crystal display device according to another aspect of the present invention comprises:
a display panel having pixels arranged in a matrix-like rows and columns configuration and having switching means connected to each of the pixels;
a row driving circuit for scanning the whole display area of the display panel by selecting rows of pixels while turning on the switching means connected thereto; and
a column driving circuit for writing signal into the pixel of the selected row in synchronization with the selection of rows;
wherein the row driving circuit subsequently selects all the rows over one field period, the column driving circuit outputs a corrected image signal when the even number row is selected and outputs an erasure signal when the odd number row is selected at the even number field, the column driving circuit outputs a corrected image signal when the odd number row is selected and outputs the erasure signal when the even number row is selected at the odd number field.
That is, this liquid crystal display device writes an erasure signal by outputting an interlaced type image signal and the erasure signal alternately to the source signal line in synchronization with selection of row. Therefore, the erasure signal can be written without largely changing the circuit structure of the conventional liquid crystal display device of the active matrix type for displaying progressive image signal.
In order to alternately output the interlaced type image signal and the erasure signal, for example, the column driving circuit is connected to the supply source of the image signal and the supply source of the erasure signal in a switchable manner, so that the connection to the supply source of the image signal and the supply source of the erasure signal may be alternately changed over for in synchronization with the row selection by the row driving circuit.
Preferably, the erasure signal to be written into each pixel is an image signal of black. In the case of a TN type liquid crystal display device of normally white mode, the response speed of the liquid crystal becomes faster in the change from white to black than in the change from black to white. The state of the liquid crystal is stabilized faster at the time of writing the erasure signal of black with an increase in the response speed of the liquid crystal.
Furthermore, the response speed of the liquid crystal is accelerated by correcting the image signal, which is applied after writing of the black erasure signal, to a corrected image signal which is enhanced in a direction of rendering the signal brighter than the original image signal. Thus, the deterioration in the screen luminance resulting from the writing of the erasure signal can be suppressed.
Furthermore, in order to further improve the display quality of the moving picture, the liquid crystal display device of the active matrix type of the present invention comprises a light source which is provided on a backside of the display panel and illuminates the display panel with dividing the display panel into a plurality of horizontal stripe display regions; and
wherein the light source illuminates the display region only for a predetermined period which is delayed from the completion of the scanning of each display region in each of the even number field and the odd number field.
Before writing the image signal, the potential of all the pixels are adjusted to the potential of the erasure signal. Since illumination is provided only in a period in which the response of the liquid crystal after writing the image signal is mostly settled, with the result that the ghost is further suppressed. Furthermore, since the illumination period is restricted to some extent, an impulse type light emission is provided so that a sharp image free from the motion blur can be provided.
In order to provide illumination divided into a plurality of display regions, a light source having a plurality of lamps which is divided for each display region and can be lighted independently can be used.
Furthermore, instead of this, a light source may be used which comprises a shutter which is divided for each display region and can be opened and closed.
As has been described above, the liquid crystal display device according to the present invention is characterized in that a voltage, with which transmittance in a steady state of the pixel with the original image signal is attained within one frame period, is determined based on the original current frame image signal and provided to liquid crystal at the current frame.
Furthermore, the liquid crystal display device according to the present invention is characterized in that a light source is provided which is capable of illuminating by dividing the display panel into regions to illuminate the region after a definite delay period after the completion of the scanning of each of the regions.
Furthermore, the liquid crystal display device according to the present invention is characterized in that a temperature of the liquid crystal in the liquid crystal display device is detected at the time of determining a voltage applied to the liquid crystal with respect to the input image signal, and a voltage is applied which is required for realizing a target transmittance after one frame in accordance with the detected temperature.
Furthermore, the liquid crystal display device according to the present invention is characterized in that a row which is originally non-selected is also scanned in each field to write an erasing signal to each pixel of the row in the case where an interlaced type image signal is displayed.